feat: 제품 검색 페이지 및 QR 라벨 인쇄 기능

- /admin/products: 전체 제품 검색 페이지 (OTC)
- /api/products: 제품 검색 API (세트상품 바코드 포함)
- qr_printer.py: Brother QL-710W 프린터 연동
- /api/qr-print, /api/qr-preview: QR 라벨 인쇄/미리보기 API
- 판매상세 페이지에 QR 인쇄 버튼 추가
- 수량 선택 UI (+/- 버튼, 최대 10장)
- 세트상품 제조사 표시 개선
- 대시보드 헤더에 제품검색/판매조회 탭 추가

backend/templates/admin.html

@@ -399,6 +399,8 @@
                 <div class="header-subtitle">청춘약국 마일리지 관리</div>
             </div>
             <div style="display:flex;gap:8px;">
+                <a href="/admin/products" style="color:rgba(255,255,255,0.9);text-decoration:none;font-size:14px;padding:8px 16px;border-radius:8px;background:rgba(255,255,255,0.15);transition:all 0.2s;">🔍 제품검색</a>
+                <a href="/admin/sales-detail" style="color:rgba(255,255,255,0.9);text-decoration:none;font-size:14px;padding:8px 16px;border-radius:8px;background:rgba(255,255,255,0.15);transition:all 0.2s;">📋 판매조회</a>
                 <a href="/admin/sales" style="color:rgba(255,255,255,0.9);text-decoration:none;font-size:14px;padding:8px 16px;border-radius:8px;background:rgba(255,255,255,0.15);transition:all 0.2s;">🧾 판매내역</a>
                 <a href="/admin/ai-crm" style="color:rgba(255,255,255,0.9);text-decoration:none;font-size:14px;padding:8px 16px;border-radius:8px;background:rgba(255,255,255,0.15);transition:all 0.2s;">🤖 AI CRM</a>
                 <a href="/admin/alimtalk" style="color:rgba(255,255,255,0.9);text-decoration:none;font-size:14px;padding:8px 16px;border-radius:8px;background:rgba(255,255,255,0.15);transition:all 0.2s;">📨 알림톡</a>